AWG to kcmil Conversion Chart and Feeder Sizing Decision Guide

The transition from American Wire Gauge (AWG) to thousands of circular mils (kcmil) occurs exactly where the AWG system runs out of numerical logic. The largest standard AWG size is 4/0 AWG (read as 'four-aught'). The moment you exceed the ampacity of 4/0 AWG, you switch to the kcmil system, starting at 250 kcmil. For a standard 75°C copper termination, 4/0 AWG is rated for 230 amps, while 250 kcmil jumps to 255 amps. If you are sizing a feeder for a 200A to 400A service, a large solar inverter, or a commercial subpanel, you will be working exclusively in the 1/0 AWG through 750 kcmil range.

The Core AWG to kcmil Sizing Table (NEC Table 310.16)

The following data is extracted directly from NEC Table 310.16 (formerly 310.15(B)(16)). This table assumes copper conductors with common high-heat insulation types like THHN, THWN-2, or XHHW-2 installed in a raceway or cable, with an ambient temperature of 30°C (86°F).

Decision Tree: Sizing Your Feeder from 100A to 400A

Do not guess your wire size based on internet forums. Use this decision path to arrive at a concrete, code-compliant pick for copper THHN/XHHW feeders protected by a standard thermal-magnetic breaker.

• IF your continuous or non-continuous load is 150A or less (e.g., a 150A subpanel feeder):
  THEN select 1/0 AWG Copper (75°C rating = 150A). Note: If the load is strictly continuous for 3+ hours, you must size the wire at 125% of the load, pushing you to 2/0 AWG.
• IF your load is exactly 200A (e.g., standard modern residential service entrance):
  THEN select 3/0 AWG Copper (75°C rating = 200A). Note: 2/0 AWG is rated for 175A and is insufficient for a 200A main breaker unless specific dwelling calculation exceptions apply.
• IF your load is 300A to 320A (e.g., large solar inverter output or EV charging hub):
  THEN select 350 kcmil Copper (75°C rating = 310A). Wait, 310A is less than 320A. If your breaker is 350A, you must step up to 400 kcmil (335A) or parallel two 1/0 AWG runs if the lugs support it.
• IF your load is 400A (e.g., 400A commercial service or heavy machinery disconnect):
  THEN select 600 kcmil Copper (75°C rating = 420A). 500 kcmil only provides 380A, which will trip a 400A breaker under full load.

Derating Large Conductors: How Bundling Modifies Your Base Value

The ampacity table above assumes no more than three current-carrying conductors in a raceway. When you pull multiple circuits through a single conduit, the heat generated by adjacent wires cannot dissipate. According to NEC Article 310.15(C)(1), you must apply an adjustment factor.

The Golden Rule of Derating: Always start your derating math using the 90°C column, but your final derated number must still be higher than your load, and you cannot exceed the 75°C column limit for termination.

What the AWG to kcmil Table Cannot Tell You

While NEC Table 310.16 prevents wires from catching fire due to overcurrent, it completely ignores two critical physical realities of large-gauge installations.

1. Voltage Drop Over Distance

The NEC does not strictly enforce voltage drop for most feeders (it only recommends a maximum of 3% for branch circuits and 5% total per Informational Note 310.15(B)). However, running 350 kcmil wire 400 feet to a detached barn will result in severe voltage sag under heavy load. To calculate this, use the Circular Mil (CM) formula: CM = (2 x K x I x D) / VD. For copper, K is roughly 12.9. If your calculated CM exceeds the physical CM of your chosen kcmil wire (e.g., 350 kcmil = 350,000 CM), you must upsize the wire purely for voltage drop, regardless of the ampacity table.

2. Physical Lug Fit and Bending Radius

A 500 kcmil wire is incredibly stiff. The table will not tell you if the wire will physically bend into the panelboard gutter. NEC Table 312.6(B) mandates minimum bending space in enclosures. For a 500 kcmil wire, you need at least 6 inches of clear space from the lug to the opposite wall. Furthermore, standard mechanical lugs have specific barrel lengths; stripping a 500 kcmil wire requires exposing exactly the right amount of conductor to seat fully in the lug without exposing bare copper, which requires specialized heavy-duty wire strippers or a meticulous utility knife technique.

Quick-Jump Reference: Most Queried kcmil Sizes

Bookmark this section for rapid jobsite lookups when dealing with the most common heavy commercial and residential service upgrades.

• 250 kcmil: The standard upsized feeder for 225A continuous loads or 250A main breakers. Often used in aluminum (where 250 kcmil Al is rated 205A at 75°C) for 200A residential services to save money over copper.
• 350 kcmil: The workhorse for 300A to 320A solar interconnects and commercial subpanels. Requires a minimum 2.5-inch conduit for three conductors plus a ground.
• 500 kcmil: Frequently paralleled in pairs (two 500 kcmil runs per phase) to achieve 400A to 600A services, as pulling a single 750 kcmil or 1000 kcmil wire is physically exhausting and requires expensive, specialized pulling equipment.

Always verify your final wire size against the specific termination temperature rating printed on your breaker or panelboard label, and torque all kcmil lugs to the exact inch-pound specification listed on the equipment using a calibrated torque wrench. A loose 350 kcmil lug will arc, melt, and fail long before the breaker ever trips.